Links an EC2-Classic instance to a ClassicLink-enabled VPC through one
or more of the VPC's security groups. You cannot link an EC2-Classic
instance to more than one VPC at a time. You can only link an instance
that's in the running state. An instance is automatically
unlinked from a VPC when it's stopped - you can link it to the VPC
again when you restart it.

After you've linked an instance, you cannot change the VPC security
groups that are associated with it. To change the security groups, you
must first unlink the instance, and then link it again.

Linking your instance to a VPC is sometimes referred to as
attaching your instance.